PCB board factory, what is the special method for designing high frequency impedance circuit boards

Characteristic impedance: Also known as "characteristic impedance", it is not a DC resistance and belongs to the concept of long-term transmission. In the high frequency range, during the signal transmission process, where the signal edge arrives, an instantaneous current will be generated between the signal line and the reference plane (power or ground plane) due to the establishment of an electric field.

If the transmission line is isotropic, then as long as the signal is transmitting, there will always be a current I, and if the output voltage of the signal is V, during the signal transmission process, the transmission line will be equivalent to a resistance, the size is V/I , and this equivalent resistance is called the characteristic impedance Z of the transmission line.

In the process of signal transmission, if the characteristic impedance of the transmission path changes, the signal will be reflected at the node with discontinuous impedance.

The factors that affect the characteristic impedance are: dielectric constant, dielectric thickness, line width, and copper foil thickness.

[1] Gradient line

Some RF devices have small packages, the width of SMD pads may be as small as 12mils, and the width of RF signal lines may be more than 50mils. Gradient lines are used, and line width sudden changes are prohibited. The gradient line is shown in the picture, and the line in the transition part should not be too long.

【2】Corner

If the RF signal line runs at a right angle, the effective line width at the corner will increase, and the impedance will be discontinuous, causing signal reflection. To reduce discontinuities, there are two ways to deal with corners: chamfering and filleting. The radius of the arc angle should be large enough. Generally speaking, it should be ensured that: R>3W. As shown on the right.

【3】Large pad

When there is a large pad on the 50-ohm microstrip line, the large pad is equivalent to distributed capacitance, which destroys the continuity of the characteristic impedance of the microstrip line. Two methods can be taken at the same time to improve: firstly, thickening the microstrip line medium, and secondly, hollowing out the ground plane under the pad, both of which can reduce the distributed capacitance of the pad. As shown below.

【4】Via

Vias are metal cylinders plated out of the through holes between the top and bottom layers of a circuit board. Signal vias connect transmission lines on different layers. Via stubs are unused parts of a via. Via pads are annular pads that connect vias to top or internal transmission lines. Isolation pads are annular voids within each power or ground plane to prevent shorts to the power and ground planes.

After rigorous physical theory derivation and approximate analysis, the equivalent circuit model of a via can be a ground capacitor connected in series at both ends of an inductor, as shown in Figure 1.

It can be seen from the equivalent circuit model that the via itself has parasitic capacitance to the ground. Assume that the diameter of the via anti-pad is D2, the diameter of the via pad is D1, the thickness of the PCB board is T, and the dielectric constant of the board substrate is ε, the parasitic capacitance of the via is approximated by:

过孔的寄生电容可以导致信号上升时间延长，传输速度减慢，从而恶化信号质量。同样，过孔同时也存在寄生电感，在高速数字PCB中，寄生电感带来的危害往往大于寄生电容。

它的寄生串联电感会削弱旁路电容的贡献，从而减弱整个电源系统的滤波效用。假设L为过孔的电感，h为过孔的长度，d为中心钻孔的直径。过孔近似的寄生电感大小近似于：

过孔是引起RF 通道上阻抗不连续性的重要因素之一，如果信号频率大于1GHz，就要考虑过孔的影响。

减小过孔阻抗不连续性的常用方法有：采用无盘工艺、选择出线方式、优化反焊盘直径等。优化反焊盘直径是一种最常用的减小阻抗不连续性的方法。由于过孔特性与孔径、焊盘、反焊盘、层叠结构、出线方式等结构尺寸相关，建议每次设计时都要根据具体情况用HFSS和Optimetrics进行优化仿真。

当采用参数化模型时，建模过程很简单。在审查时，需要PCB设计人员提供相应的仿真文档。

过孔的直径、焊盘直径、深度、反焊盘，都会带来变化，造成阻抗不连续性，反射和插入损耗的严重程度受影响。

【5】通孔同轴连接器

与过孔结构类似，通孔同轴连接器也存在阻抗不连续性，所以解决方法与过孔相同。减小通孔同轴连接器阻抗不连续性的常用方法同样是：采用无盘工艺、合适的出线方式、优化反焊盘直径。